1. Field of the Invention
This invention relates to an electroluminescent device which has utility in various types of display devices, and more particularly, to an organic electroluminescent device which works at low drive voltage and has a high luminance and good stability.
2. Description of the Prior Art
Electroluminescent devices are self-luminescent in nature, and are thus brighter than liquid crystal devices, ensuring a clear display. Accordingly, research works have long studied such devices. Existing electroluminescent devices, which arrive at a level in practical use, are those which make use of inorganic materials such as ZnS. However, such inorganic electroluminescent devices are not in wide use because their drive voltage for light emission is as high as 200 V or over.
On the contrary, organic electroluminescent devices, which utilize light-emitting organic materials, have still been far from the practical level. In 1987, C. W. Tang et al of Eastman Kodak Co., Ltd. developed a built-up structure device, with a drastic advance in characteristic performance. More particularly, the device has a built-up structure which includes a fluorescent body having a stable deposited film and capable of transporting electrons, and an organic film capable of transporting holes. The carrier from the both are charged into the fluorescent body, with a success in light emission. This organic electroluminescent device has a much improved luminescent efficiency, and is capable of emission at 1000 cd/m.sup.2 or more on application of a voltage of 10 V or below. Thereafter, many researchers have made studies on the improvements of characteristics. At present, a light-emitting characteristic of 10,000 cd/m.sup.2 or over is obtained.
The fundamental light-emitting characteristic of the organic device has been within a satisfactory serviceable range, but the most serious problem, which should be overcome in order to realize the practical use of the organic device, resides in the shortage of stability. More particularly, the organic electroluminescent device is disadvantageous in that its emission luminance lowers, and a non-light-emitting region, usually called "dark spot", appears, with their area being enlarged from the spot or spots.
The dark spot is caused and facilitated to develop in the device by the presence of moisture entering from defective portions thereof. In order to suppress moisture from entering, there has been proposed the provision of a sealing layer covering the device therewith (Japanese Laid-open Patent Application Nos. 4-267097 and 7-192866). In the Japanese Laid-open Patent Application No. 4-267097, a first film is formed according to a vapor phase method, such as a vacuum deposition technique, on which a photo-curable resin is coated and cured in situ to form a second layer, thereby forming a built-up protective structure. It is stated that when the protective structure is formed in this way, damages, which are caused at the time of the formation of the second layer, can be mitigated. It is also stated in this publication that this structure is effective in suppressing degradation of an emission intensity. The sealing, protective layer disclosed in the Japanese Laid-open Patent Application No. 7-192866 is one which is made of a mixture of an organic component as used in an organic compound layer in an electroluminescent device and an inorganic oxide. It is set out that this layer is able to suppress the occurrence of dark spot. However, in either case, it is not always satisfactory with respect to the enlargement of a dark spot. This presents a serious problem when the organic electroluminescent device is in service.